Tires exhibit an internal surface of low permeability to air in order to prevent deflation of the tire and to protect sensitive internal regions of the latter from admissions of oxygen and water, such as plies comprising oxidation-sensitive metal cables, this protection making it possible to improve the endurance of the tire. Today, such protection of the internal surface of the tires is provided by inner liners composed of elastomeric compositions based on butyl rubber.
However, since savings in fuel and the need to protect the environment have become a priority, it is desirable to produce inner liners which are impermeable to air and which exhibit a weight and a hysteresis which are as low as possible, in order to obtain an improved rolling resistance of the tire. In point of fact, the performance in terms of impermeability to air of butyl rubbers is related to a not insignificant minimum thickness (of the order of a millimeter) and thus to a certain weight, which does not make it possible to efficiently respond to these new requirements.
Thus, it is necessary to add reinforcing fillers, such as carbon black, to the inner liner elastomeric composition in order to improve its impermeability. However, in a large amount, these reinforcing fillers have a detrimental effect on some properties of the composition, both in the raw state: difficulty in working the raw composition, commonly known as “processibility”, and in the cured state: deterioration in mechanical properties, in particular reduction in the flexural strength. The introduction of plasticizer of oil type makes it possible to overcome these aspects of working and of mechanical properties but is very highly disadvantageous to the impermeability.
Various solutions have been envisaged in order to overcome these disadvantages, in particular by resorting to other types of fillers which are additional to the reinforcing fillers, often known under the name of smectites and in particular of organophilic smectites. These organophilic smectites improve the impermeability properties of the materials if they are well dispersed in the material, that is to say both a homogeneous distribution of these fillers in the material and good compatibility with the latter. This dispersion is often difficult to obtain due to the low thermodynamic compatibility existing between the elastomers and such fillers.
The publication JP 2004143366A describes a tire inner liner composition based on butyl rubber which comprises carbon black and which also comprises graphite dispersed in the elastomeric matrix, the addition of this graphite making it possible to improve the impermeability to gases. However, the fact of adding an additional filler is harmful to the processibility of the mixture.
Commonly assigned French patent application number 07/03751, filed 25 May 2007, not yet published to date, discloses adding graphite and a resin having a glass transition temperature of greater than 30° C. which makes it possible both to improve the processibility of the mixture (with regard to a mixture not comprising graphite or comprising graphite alone) and to improve the impermeability of the composition (with regard to a mixture not comprising graphite and with regard to a mixture comprising graphite alone).
The assignee has continued these research studies and has discovered, surprisingly, that the combination, in a rubber composition for a tire inner liner, of a polyisobutylene oil of low molecular weight with graphite, in the presence of the elastomer conventionally used, such as butyl rubber, and of a reinforcing filler, makes it possible not only to obtain properties of processibility and of flexural strength which are as good as those of the compositions of the prior art but also to obtain improved properties of impermeability to gases and to significantly improve the properties of rolling resistance and of endurance without, of course, this being at the expense of other properties.
As was mentioned above, oils are known for their property of improving the processibility but it is also known that they can be harmful to the leaktightness. It has been found here, surprisingly, on the one hand, that the graphite and low weight polyisobutylene oil combination makes it possible to improve the leaktightness, including with respect to a composition including only graphite, and, on the other hand, that this composition also makes it possible, at the same leaktightness with respect to the graphite/resin solution (abovementioned commonly assigned French application), to lower the hysteresis of the mixture and thus to lower the rolling resistance of a tire having an inner liner made from this composition.